Acrylic acid esters are prepared by an esterification reaction between acrylic acid and an alcohol. Inorganic acids, organic acids and solid acids are used as a catalyst for the esterification reaction. When one uses the inorganic acid catalyst (e.g., sulfuric acid) that is difficult to separate, the acid catalyst component in a product stream after the reaction is separated by adding a counter base such as sodium hydroxide and thus converting it into a salt.
Because such a treatment for separating the acid catalyst lays a burden on the process and the environment, it is preferable to adopt a process employing the organic acid or the solid acid which is easy to separate. In the case of the solid acid, a reaction entails a tendency of (mechanical, thermal and chemical) inactivation of the catalyst so that separation or a supplement of the catalyst is necessary and a relatively exacting process for separating the solids is required. The organic acids have an advantage that the recirculation of the catalyst is comparatively simple as they are easy to separate in the product stream.
In most processes for the esterification for acrylic acid esters, besides an effective product of the esterification reaction, various heavy byproducts such as the Michael adducts are generated from side reactions such as the Michael addition.
For example, in a process for preparing butyl acrylate, representative byproducts include butyl-b-butoxy propionate (BPB), b-butoxypropionic acid (BPA) and n-butyl diacrylate (BDA). Hereinafter, such byproducts are referred to as the “Michael Adducts.” Although it is possible to minimize the occurrence of the side reactions by optimizing reaction conditions for the esterification, these byproducts are inevitably generated in almost all processes. To deal with such problems, prior arts proposed effective methods for decomposing and recovering these byproducts.
For these recovery methods to be successfully applied in the field, one should consider many aspects such as economic feasibility regarding the cost for the catalyst, corrosion of equipment and the disposal of waste steams after the final treatment.
Japanese Patent No. 1993-025086 suggested a process for decomposing the Michael adducts by adding an excessive amount of water with using sulfuric acid as a catalyst. However, the suggested process has drawbacks that it shows a low conversion rate of about 30% and consumes a lot of energy for the same reaction due to using an excessive amount of water.
U.S. Pat. No. 5,734,075 (issued in 1998) proposed a process of thermal cracking in the absence of catalyst on the byproducts from the esterification with the addition of a distillation residue stream originating from acrylic acid. It says that adding acrylic dimers or oligomers to the byproducts of the esterification process makes it possible to ensure the fluidity of the residue stream and the fouling phenomenon can be reduced as the process is operated in the absence of catalyst.
However, because this technique does not use any catalyst, the reactivity of the cracking is relatively low as compared with a process using a catalyst. Therefore, the conversion rate of about 80% at a considerably high temperature (280° C.) is required in order to obtain a high conversion rate, which renders the process economically unfavorable.
U.S. Pat. No. 5,910,603 (issued in 1999) described a process for catalytically decomposing the Michael adducts originating from the esterification of acrylic acid with using an organic or inorganic acid.
When using an organic acid catalyst, this process exhibits the conversion rate of about 80% at a temperature between 150° C. and 250° C. but it still needs a high temperature. Also, the process could not deal with a serious fouling problem in a residue stream after the cracking reaction and from the viewpoint of applying the process, leaching of the catalytic component remains unresolved.
In order to solve the fouling problem and the leaching of solids in the prior art, U.S. Pat. No. 6,617,470 (issued in 2003) adopts an alkylbenzene sulfonic acid with a longer chain than pTSA as an organic acid catalyst for use in the cracking process. As this process does not use pTSA, a fluidity of waste oils can be ensured. Disadvantageously, however, this process should additionally add and use a long chain alkylbenzene sulfonic acid that is relatively expensive and shows a low reactivity for a cracking process, or directly use it as a catalyst for the esterification reaction.